1. Field of the Invention
The present invention relates to a laser beam spot shape detecting method of detecting the spot shape of a laser beam oscillated by laser beam oscillating means and focused by focusing means in a laser processing apparatus.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer thus formed is cut along the division lines to thereby divide the regions where the devices are formed from each other, thus obtaining the individual devices. Further, an optical device wafer is provided by forming a gallium nitride compound semiconductor or the like on the front side of a sapphire substrate or a silicon carbide substrate. The optical device wafer is also cut along the division lines to obtain individual optical devices divided from each other, such as light emitting diodes and laser diodes, which are widely used in electric equipment.
As a method of dividing such a wafer as mentioned above along the division lines, there has been proposed a method including the steps of applying a pulsed laser beam having an absorption wavelength to the wafer along the division lines to thereby form laser processed grooves as a break start point on the wafer along the division lines and next applying an external force to the wafer along the division lines where the laser processed grooves are respectively formed, thereby breaking the wafer along the division lines (see Japanese Patent Laid-open No. Hei 10-305420, for example).
As another method of dividing such a wafer as mentioned above along the division lines, there has been tried a dividing method using a laser processing method including the step of applying a pulsed laser beam having a transmission wavelength to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in an area to be divided. The dividing method using this laser processing method includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer, thereby continuously forming modified layers inside the wafer along the division lines and next applying an external force along the division lines where the modified layers are respectively formed to be reduced in strength, thereby breaking the wafer along the division lines (see Japanese Patent No. 3408805, for example).
However, focusing means for focusing the laser beam oscillated by laser beam oscillating means is configured by a combination lens composed of a plurality of convex lenses and concave lenses. Further, an optical system provided between the laser beam oscillating means and the focusing means has distortion. Accordingly, the shape of a focused spot on the wafer does not always become an intended shape such as a circular shape. It is known that the shape and size of the focused spot of a laser beam have an influence on the quality of laser processing. Therefore, the shape and size of the focused spot of a laser beam to be applied to a workpiece such as a wafer are detected.